Method and apparatus for reducing specular reflections in semiconductor lasers

ABSTRACT

The present invention is directed a method of fabricating a VCSEL. First, a substrate with a back surface and a front surface is provided. Then, a first reflector, an active region, and a second reflector are disposed on the front surface. The first reflector is disposed on the front surface. The active region is interposed between the first reflector and the second reflector. Then, anti-reflection features are formed into the back surface of the substrate to reduce specular reflection of light into the active region.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to devices that emitelectromagnetic radiation and, in particular, to vertical-cavitysurface-emitting lasers (VCSELs).

[0003] 2. Description of the Related Art

[0004] The following descriptions and examples are not admitted to beprior art by virtue of their inclusion within this section.

[0005] Laser beams are reflected to some extent from any surfacecontacted. If the reflected rays remain parallel (i.e., the angle ofreflection equals the angle of incidence), the reflection is called“specular”. This means that the rays striking the surface are reflectedfrom the surface according to the law of reflection, qi=qr. If thereflected rays are randomly scattered, the reflection is called“diffuse”. Specular reflections are produced by highly-polished,mirror-like surfaces whereas diffuse reflections result from rough,irregular surfaces (however, specular reflections can also be producedby rough surfaces when the size of the surface irregularities is lessthan the wavelength of the incident radiation). The distinction betweena specular reflection and a diffuse reflection is not always clearlydefined. Except for reflections from precisely constructed opticalmirrors, all beams are to some extent divergent. In general, however,the rougher the reflecting surface, the greater will be the divergence(or diffuseness) of the reflected beam. FIG. 1 illustrates various typesof laser reflections. FIG. 2 illustrates specular reflection from apolished mirror surface. FIG. 3 illustrates diffuse reflection from acorrugated surface.

[0006] Lasers have a wide range of industrial and scientific uses. Thereare several types of lasers, including gas lasers, solid-state lasers,liquid (dye) lasers, and free electron lasers. Semiconductor lasers arealso in use. The possibility of amplification of electromagnetic wavesin a semiconductor superlattice structure, i.e., the possibility ofsemiconductor diode lasers, was predicted in a seminal paper by R. F.Kazarinov, et al., “Possibility of the Amplification of ElectromagneticWaves in a Semiconductor with a Superlattice,” Soviet PhysicsSemiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductorlaser technology has continued to develop since this discovery.

[0007] There are a variety of types of semiconductor lasers.Semiconductor lasers may be diode lasers (bipolar) or non-diode laserssuch as quantum cascade (QC) lasers (unipolar). Semiconductor lasers ofvarious types may be electrically pumped (by a DC or AC current), orpumped in other ways, such as by optically pumping (OP) or electron beampumping. Semiconductor lasers are used for a variety of applications andcan be built with different structures and semiconductor materials, suchas gallium arsenide (GaAs).

[0008] Semiconductor lasers are typically powered by applying anelectrical potential difference across the active region, which causes acurrent to flow therein. Electrons in the active region attain highenergy states as a result of the potential applied. When the electronsspontaneously drop in energy state, photons are produced (to carry awaythe energy lost by the transition, so as to conserve energy). Some ofthose photons travel in a direction perpendicular to the reflectors ofthe laser. As a result of the ensuing reflections, the photons cantravel through the active region multiple times.

[0009] Stimulated emission occurs when an electron is in a higher energylevel and a photon with an energy equal to the difference between theelectron's energy and a lower energy interacts with the electron. Inthis case, the photon stimulates the electron to fall into the lowerenergy state, thereby emitting a photon. The emitted photon will havethe same energy as the original photon, and, if viewed as waves, therewill be two waves emitted (from the electron's atom) in phase with thesame frequency. Thus, when the photons produced by spontaneous electrontransition photons interact with other high energy state electrons,stimulated emission can occur so that two photons with identicalcharacteristics are present. If most electrons encountered by thephotons are in the high energy state, the number of photons travelingbetween the reflectors tends to increase, giving rise to amplificationof light and thus lasing.

[0010] The use of semiconductor diode lasers (both edge-emitting andsurface-emitting) for forming a source of optical energy is attractivefor a number of reasons. For example, diode lasers have a relativelysmall volume and consume a small amount of power as compared toconventional laser devices. Further, the diode laser is a monolithicdevice, and does not require a combination of a resonant cavity withexternal mirrors and other structures to generate a coherent outputlaser beam.

[0011] Additionally, semiconductor lasers may be edge-emitting lasers orsurface-emitting lasers (SELs). Edge-emitting semiconductor lasersoutput their radiation parallel to the wafer surface, while in SELs, theradiation output is perpendicular to the wafer surface.

[0012] One type of SEL is the vertical-cavity surface-emitting laser(VCSEL). The VCSEL structure usually consists of an active (opticalgain) region sandwiched between two distributed Bragg reflector (DBR)mirrors: a top, exit DBR, and a bottom DBR. DBRs are sometimes referredto as mirror stacks. The DBR mirrors of a typical VCSEL can beconstructed from dielectric or semiconductor layers (or a combination ofboth, including metal mirror sections). DBRs or mirror stacks in VCSELsare typically formed of multiple pairs of layers often referred to asmirror pairs. The pairs of layers are formed of a material systemgenerally consisting of two materials having different indices ofrefraction and being easily latticed matched to the other portions ofthe VCSEL. The number of mirror pairs per stack may range from 20-40pairs to achieve a high percentage of reflectivity, depending on thedifference between the refractive indices of the layers. A larger numberof mirror pairs increases the percentage of reflected light(reflectivity).

[0013] When properly designed, these mirror pairs will cause a desiredreflectivity at the laser wavelength. Typically in a VCSEL, the mirrorsare designed so that the bottom DBR mirror (i.e. the one interposedbetween the substrate material and the active region) has nearly 100%reflectivity, while the top DBR mirror has a reflectivity that may be98%-99.5% (depending on the details of the laser design). Of course,various laser structures may vary from these general properties.

[0014] High reflectivity (approaching 100%) at the bottom DBR mirror isgenerally desired in a VCSEL for two reasons. First, any portion of theoptical field that “leaks” out the back of the bottom DBR mirrorrepresents a power loss that reduces efficiency. This reduced efficiencymay be so great so as to prevent the laser from operating at all (i.e.the efficiency goes to zero). A second reason why nearly unityreflection coefficient is desired for the bottom DBR mirror is relatedto the issue of optical feedback into the laser cavity.

[0015] From the standpoint of optics, any change in the index ofrefraction along the path of a light ray can be interpreted as a mirror,i.e., any change in the refractive index at an interface will cause somelight to be reflected from the surface, rather than transmitted through.For the simple case of a ray of light normally incident to asemiconductor, the proportion of the light intensity reflected back fromthe interface is given by the equation:$R = \frac{\left( {n - 1} \right)^{2} + k^{2}}{\left( {n + 1} \right)^{2} + k^{2}}$

[0016] where n is defined as the index of refraction that describes thesemiconductor optical properties, while k is the extinction coefficient.Any light that is not reflected by the bottom DBR mirror will betransmitted through the semiconductor for some distance (if k is zerofor the substrate material, then it will be transmitted essentiallywithout loss). Eventually, the light will impinge on the back surface ofthe semiconductor substrate, where it will undergo a specularreflection, according to the above equation. This specular reflectionwill cause some light to be reflected back towards the bottom DBRmirror, thereby forming a fabry-perot cavity (or etalon) between theback surface of the semiconductor substrate and the bottom DBR mirror.This etalon will inevitably couple with the laser cavity itself so as toaffect the stable modes of the laser cavity. This effect may causeundesirable instabilities in the laser operation, such as mode-hopping,in which the laser optical field oscillates between two competing cavitymodes. Other dynamic effects may also adversely affect the ability ofthe laser to be rapidly switched on and off, which may limit theapplication of the laser for some purposes, such as telecommunicationsor laser-based spectroscopic sensing.

[0017] For this reason, it is desirable to be able to limit the specularreflection of light from the back surface of the semiconductor substratein a VCSEL.

[0018] One possible way to limit this specular reflection of light is toselect an absorbing material for the semiconductor substrate so that anylight that leaks out of the laser cavity through the bottom DBR mirrorwill be absorbed nearly completely before reaching the back surface ofthe substrate. However, the choice of substrate is already constrainedby many factors in the laser design, such as lattice-matching to the DBRand active region materials, and electrical requirements such as thenecessity to provide adequate doping to allow current to flow throughthe substrate material. In addition, since the substrate itself is oftenonly several thousandths of an inch thick (typically 20 mils or 500micrometers), it may be difficult to select a material with asufficiently high absorption to reduce the amount of specular reflectionto an acceptable level.

[0019] There is therefore a need for improved methods and apparatus forreducing specular reflection of light into the laser cavity of a VCSEL.

SUMMARY OF THE INVENTION

[0020] The present invention is directed to a VCSEL and method offabricating same. First, a substrate with a back surface and a frontsurface is provided. Then, a first reflector, an active region, and asecond reflector are disposed on the substrate. The first reflector isdisposed on the front surface. The active region is interposed betweenthe first reflector and the second reflector. Then, anti-reflectionfeatures are formed into the back surface of the substrate to reducespecular reflection of light into the active region and the lasercavity.

[0021] Another embodiment of the present invention is directed to aVCSEL that includes a substrate with a back surface and a front surface.It also includes a first reflector disposed on the front surface of thesubstrate, an active region disposed on the first reflector, and asecond reflector disposed on the active region such that the activeregion is interposed between the first reflector and the secondreflector. The back surface of the substrate includes anti-reflectionfeatures for reducing specular reflection of light into the activeregion.

[0022] Another embodiment is directed to an array of VCSELs, each VCSELhaving: a substrate with a back surface and a front surface; a firstreflector disposed on the front surface of the substrate; an activeregion disposed on the first reflector; and a second reflector disposedon the active region such that the active region is interposed betweenthe first reflector and the second reflector. The back surface of thesubstrate comprises anti-reflection rows that have substantiallytriangular cross-sections. The angles of the substantially triangularcross-sections are arranged to reflect light away from the active regionof each VCSEL.

[0023] Another embodiment in accordance with the present inventiondiscloses a method of conditioning a semiconductor substrate. The methodincludes the step of forming anti-reflection features on the backsurface of the substrate.

[0024] Another embodiment in accordance with the present inventiondiscloses an apparatus for conditioning the back surface of asemiconductor substrate in the presence of an etching solution. Theapparatus includes a line-focused laser beam generator for generatingand applying a line-focused laser beam to the back surface of thesemiconductor substrate; a holder positioned under the line-focusedlaser beam generator, in which the holder holds the semiconductorsubstrate in place while the semiconductor substrate is beingconditioned; and a controller communicably linked to the line-focusedlaser beam generator, in which the controller controls the relativemovement between the line-focused laser beam generator and thesemiconductor substrate on the holder so as to form anti-reflectionfeatures on the back surface of the semiconductor substrate.

[0025] An advantage of the present invention is that it reduces theamount of specular reflection of light from the back surface of thesemiconductor substrate into the active region of the VCSEL.

[0026] Another advantage is that a VCSEL in accordance with the presentinvention will have more stable modes of laser cavity than itsconventional peer, thus enabling a more stable laser operation.

[0027] Another advantage of the present invention is that it reducesmode hopping in the laser cavity of the VCSEL.

[0028] Other and further features and advantages will be apparent fromthe following description of presently preferred embodiments of theinvention, given for the purpose of disclosure and taken in conjunctionwith the accompanying drawings. Not all embodiments of the inventionwill include all the specified advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Other features and advantages of the invention will becomeapparent upon study of the following description, taken in conjunctionwith the attached FIGS. 1-7.

[0030]FIG. 1 illustrates various types of laser reflections;

[0031]FIG. 2 illustrates specular reflection from a polished mirrorsurface;

[0032]FIG. 3 illustrates diffuse reflection from a corrugated surface;

[0033]FIG. 4 illustrates a cross-sectional view of a one-dimensionalarray of VCSELs;

[0034]FIG. 5 is an isometric view of the one-dimensional array of VCSELsof FIG. 4;

[0035]FIG. 6 illustrates a schematic diagram of an array of VCSELs inaccordance with an embodiment of the present invention; and

[0036]FIG. 7 illustrates a schematic diagram of an apparatus forconditioning a semiconductor substrate in accordance with an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] Referring now to the drawings, the details of preferredembodiments of the invention are schematically illustrated.

[0038] As noted above, the subject of this invention is a VCSEL andmethod for fabricating same so as to reduce specular reflection of lightfrom the back surface of the semiconductor substrate that couples intothe laser cavity of a VCSEL. Referring now to FIG. 4, a one-dimensionalarray 40 of VCSELs is shown in cross-section, the cross-sectional viewillustrating three VCSELs of VCSEL array 40. Array 40 is aone-dimensional (linear) array having a single row of N VCSELs. As willbe appreciated, a VCSEL is a semiconductor laser that emits its outputperpendicular to its p-n junction. Each VCSEL is built on a substrate 12of semiconductor material. Various semiconductor materials known tothose in the art can be employed.

[0039] Each VCSEL shares a first reflector 14. In one embodiment, thefirst reflector 14 comprises a DBR. A DBR consists of alternating layersof different semiconductors or different dielectrics. In one embodiment,forty alternating layers yield a reflectivity of 99.99%.

[0040] Each VCSEL also shares the common active region 16 of VCSEL array40. The active region 16 is a p-n junction and the width of the activeregion 16 controls the wavelength of emitted light. The widths of thefirst reflector 14, active region 16, and second reflector 18 are verysmall compared to the width of the substrate 12. (The widths shown forthe various layers in FIG. 4 are not to scale.)

[0041] In the embodiment shown in FIG. 4, the second reflector 18 isseparate for each VCSEL. In another embodiment, the VCSELs can share acommon second reflector 18. In an embodiment, the second reflector 18comprises a DBR. The second reflector 18 is highly reflective, butpreferably less reflective than the first reflector 14. In oneembodiment, twenty-five alternating layers of a DBR yield a reflectivityof 99.9%.

[0042] A cladding 20 protects the VCSEL structure. Adjacent the secondreflector 18, the cladding 20 defines a gap 22 that allows emission ofradiation from the active region 16. The profile of the emittedradiation can be controlled by modifying the geometry of the emissionarea. For example, changes in the shape and size of the gap 22 affectthe spatial profile of the emitted radiation. The distance between theactive region 16 and the gap 22 also affects the spatial profile of theemitted radiation. The use of photolithographic techniques in definingthe features of the VCSEL array allow highly accurate placing of theVCSELs and highly accurate definition of VCSEL output profiles.

[0043] Referring now to FIG. 5, an isometric view of a linear array 40of VCSELs is depicted. Most of the structural aspects of the VCSELs arenot visible from an outside view. The gaps 22 are visible and allowradiation to be emitted. As with FIG. 4, the dimensions have beenrendered disproportional in order to make visible the various features.In various embodiments, the VCSEL separation and height are much reducedrelative to the thickness of the substrate. FIG. 5 shows a linear array40 of VCSELs manufactured such that radiation from each VCSEL is emittedalong substantially parallel paths. While the array shown has threeVCSELs, other embodiments include arrays having a large number oflinearly arranged VCSELs.

[0044] Referring now to FIG. 6, a cross-sectional view of an array ofVCSELs 100 in accordance with an embodiment of the present invention isillustrated. Array 100 is a one-dimensional (linear) array having asingle row of N VCSELs. Each VCSEL is built on a substrate 110 ofsemiconductor material.

[0045] Each VCSEL, e.g., VCSEL 160, shares a first reflector 120. In oneembodiment, the first reflector 120 comprises a DBR. Each VCSEL alsoshares the common active region layer 130 of VCSEL array 100. Activeregion layer 130 contains several VCSEL active regions. The widths ofthe first reflector 120, active region layer 130, and the secondreflector 140 are very small compared to the width of the substrate 110.(The widths shown for the various layers in FIG. 6 are not to scale.) Inthe embodiment shown in FIG. 6, each VCSEL, e.g., VCSEL 160, also sharesa common second reflector 140. In another embodiment, the secondreflector 140 is separate for each VCSEL. Like the first reflector 120,the second reflector 140 can comprise a DBR.

[0046] As illustrated in FIG. 6, anti-reflection features 150 are formedon the back surface of the semiconductor substrate 110 to reducespecular reflection of light into the active region of each VCSEL. Inone embodiment, the anti-reflection features 150 are specificallydesigned to reflect light off the back surface of the substrate 110 awayfrom the active region of each VCSEL, e.g., VCSEL 160, thus reducing theundesired coupling of the specular reflection of light into the activeregion of each VCSEL. In an embodiment, the anti-reflection features 150are anti-reflection rows. In accordance with another embodiment, theanti-reflection rows have periodical and geometric cross-sections, suchas triangular, sinusoidal, etc. The geometric cross-sections can be inany shape so long as they reduce specular reflection of light into theactive region or laser cavity of the VCSEL.

[0047] In one embodiment, the anti-reflection features 150 areanti-reflection rows with substantially triangular cross-sections. Inaccordance with another embodiment, the rows are designed to only haveone to two triangular cross-sections under each VCSEL, e.g., VCSEL 160.The triangular cross-sections may be of any form of triangle, such asisosceles, equilateral, right triangles, etc. In an embodiment, theangles of the triangular cross-sections are designed such that light isreflected away from the VCSELs in the array 100. The angles of thetriangular cross-sections may also be designed to reduce the amount ofspecular reflection of light in parallel with the light generated in theactive region.

[0048] In one embodiment, the anti-reflection features 150 creatediffuse reflection of light as opposed to specular reflection of light.In yet another embodiment, the anti-reflection features 150 aremechanically designed such that the specular reflection of light fromthe back surface of the substrate 110 impinges on the active regionlayer 130 outside of the active (pumping) region for each VCSEL, i.e.,the specular reflection of light will be efficiently absorbed. Thepresent invention may be utilized regardless of the pumping scheme(e.g., optical or electrical) employed in the VCSEL.

[0049] The anti-reflection features 150 may be formed on the backsurface of the substrate 110 by various methods. These features 150 maybe formed, e.g., mechanically or chemically. In accordance with anembodiment of the present invention, the anti-reflection features 150are formed into the back surface of the substrate by etching thefeatures 150 into the back surface using a selective wet etchingsolution. Such a solution would preferentially etch the semiconductorsubstrate 110 along one crystal axis. In this way, a substantiallytriangular cross-section would be created, as illustrated in FIG. 6. Theangles of the triangle would be determined by the etch chemistry usedand the relative rate of etch along the various crystal planes. Theseangles could not be altered without altering the etching chemistryand/or orientation of the VCSELs with respect to the substrate crystalplanes.

[0050] In accordance with another embodiment of the present invention,the anti-reflection features 150 are etched by a photoelectrochemicalprocess. In this process, the substrate is typically in the presence ofan etching solution, e.g., immersed in an etching solution in a vat, andthe rate of etching is enhanced (i.e. augmented) by the presence oflight. Under certain conditions the etching rate can be controlledwithin a range by varying the intensity of the light so that the regionswith higher light intensity would etch faster than those with lowerlight intensity. In this way, appropriate optics could be designed tocreate the desired pattern of light intensity so that the desiredetching profile is created. Various photoelectrochemical etchingtechniques are discussed in Ostermayer, et al. “Photoelectrochemicaletching of integral lenses on InGaAsP/InP light-emitting diodes,” Appl.Phys. Lett., Vol. 43, No. 7, Oct. 1, 1983, pp. 642-644. This article isincorporated herein by reference.

[0051] Another embodiment of the present invention discloses one type ofphotoelectrochemical etching, in which the anti-reflection features 150are etched on the back surface by scanning the back surface with aline-focused laser beam whose speed is varied by a computer. Inaccordance with another embodiment, the back surface is exposed to anetching solution prior to scanning it with the line-focused laser beam.In the substrate regions where the line is scanned the fastest, theamount of material etched away is lowest, while in the substrate regionswhere the speed is the slowest, the etching rate is the highest. Thatis, the anti-reflection features 150 have the deepest cross-sectionswhen the speed is slowest and the shallowest cross-sections when thespeed is fastest. Thus, by sinusoidally varying the scanning speed ofthe beam, for example, a substantially sinusoidal etch profile iscreated. In accordance with an embodiment of the present invention, theanti-reflection features 150 have arbitrary linearly symmetriccross-sections.

[0052] As shown in FIG. 7, an apparatus for conditioning a semiconductorsubstrate 110 in accordance with this embodiment includes a line-focusedlaser beam generator 170 for generating and applying a line-focusedlaser beam to the back surface of the semiconductor substrate 110 (inthe presence of an etching solution) and a device 180 that holds thesemiconductor substrate in place while the semiconductor substrate 110is being conditioned. The holder device 180 is typically positionedunder the line-focused laser beam generator. In one embodiment, theholder device 180 includes a vat filled with an etching solution suchthat the back surface of the substrate 110 is in the presence of theetching solution. The apparatus further includes a controller 190 forcontrolling the relative movement between the line-focused laser beamgenerator 170 and the semiconductor substrate 110 so as to formanti-reflection features 150 on the back surface of the semiconductorsubstrate 110. In one embodiment, the controller 190 moves the holderdevice 180, using a motor 195 under the control of the controller 190,along with the semiconductor substrate 110 positioned therein relativeto the line-focused laser beam generator 170. In another embodiment, thecontroller 190 moves the line-focused laser beam generator 170 relativeto the semiconductor substrate 110 positioned in the holder device 180.

[0053] Although the present invention is described with respect to anarray 100 of VCSELs, the present invention is equally applicable to asingle VCSEL.

[0054] The present invention, therefore, is well adapted to carry outthe objects and attain the ends and advantages mentioned, as well asothers inherent therein. While the invention has been depicted anddescribed and is defined by reference to particular preferredembodiments of the invention, such references do not imply a limitationon the invention, and no such limitation is to be inferred. Theinvention is capable of considerable modification, alteration andequivalents in form and function, as will occur to those ordinarilyskilled in the pertinent arts. The depicted and described preferredembodiments of the invention are exemplary only and are not exhaustiveof the scope of the invention. Consequently, the invention is intendedto be limited only by the spirit and scope of the appended claims (ifany), giving full cognizance to equivalents in all respects.

What is claimed is:
 1. A method for fabricating a vertical-cavitysurface-emitting laser (VCSEL), comprising the steps of: providing asubstrate having a back surface and a front surface, the front surfacehaving disposed thereon a first reflector, an active region, and asecond reflector, the first reflector being disposed on the frontsurface, the active region being interposed between the first reflectorand the second reflector; and forming anti-reflection features into theback surface of the substrate to reduce specular reflection of lightinto the active region.
 2. The method of claim 1, wherein theanti-reflection features are anti-reflection rows.
 3. The method ofclaim 1, wherein the step of forming anti-reflection features comprisesthe step of etching the anti-reflection features into the back surfaceof the substrate.
 4. The method of claim 3, wherein the step of etchinganti-reflection features comprises using a selective wet etchingsolution to etch the anti-reflection features.
 5. The method of claim 2,wherein the anti-reflection rows have substantially triangularcross-sections.
 6. The method of claim 5, wherein the angles of thesubstantially triangular cross-sections are designed to reduce theamount of specular reflection of light in parallel with the lightgenerated in the active region.
 7. The method of claim 3, wherein thestep of etching anti-reflection features comprises usingphotoelectrochemical etching process to etch the anti-reflectionfeatures.
 8. The method of claim 7, wherein the step of usingphotoelectrochemical etching process comprises scanning the back surfaceof the substrate with a line-focused laser.
 9. The method of claim 8,wherein the anti-reflection features have the deepest cross sectionswhen the speed of the line-focused laser is slowest.
 10. The method ofclaim 2, wherein the anti-reflection rows have arbitrary cross-sections.11. A VCSEL comprising: a substrate having a back surface and a frontsurface; a first reflector disposed on the front surface of thesubstrate; an active region disposed on the first reflector; and asecond reflector disposed on the active region such that the activeregion is interposed between the first reflector and the secondreflector, wherein the back surface of the substrate comprisesanti-reflection features for reducing specular reflection of light intothe active region.
 12. The VCSEL of claim 11, wherein theanti-reflection features are anti-reflection rows.
 13. The VCSEL ofclaim 11, wherein the anti-reflection features are etched on the backsurface using a selective wet etching solution.
 14. The VCSEL of claim12, wherein the anti-reflection rows have substantially triangularcross-sections.
 15. The VCSEL of claim 14, wherein the angles of thesubstantially triangular cross-sections are designed to reduce theamount of specular reflection of light in parallel with the lightgenerated in the active region.
 16. The VCSEL of claim 11, wherein theanti-reflection features are etched on the back surface byphotoelectrochemical etching.
 17. The VCSEL of claim 16, wherein theanti-reflection features are etched on the back surface by scanning theback surface with a line-focused laser.
 18. The VCSEL of claim 17,wherein the anti-reflection rows have the deepest cross sections whenthe speed of the line-focused laser is slowest.
 19. The VCSEL of claim12, wherein the anti-reflection rows have arbitrary cross-sections. 20.An array of VCSELs, each VCSEL comprising: a substrate having a backsurface and a front surface; a first reflector disposed on the frontsurface of the substrate; an active region disposed on the firstreflector; and a second reflector disposed on the active region suchthat the active region is interposed between the first reflector and thesecond reflector, wherein the back surface of the substrate comprisesanti-reflection features for reducing specular reflection of light intothe active region of each VCSEL.
 21. The array of claim 20, wherein theanti-reflection features are anti-reflection rows.
 22. The array ofclaim 21, wherein the anti-reflection rows have substantially triangularcross-sections.
 23. The array of claim 22, wherein the angles of thesubstantially triangular cross-sections are arranged to reflect lightaway from the active region of each VCSEL.
 24. A method for conditioninga semiconductor substrate having a back surface, the method comprisingthe step of forming anti-reflection features on the back surface of thesubstrate.
 25. The method of claim 24, wherein the anti-reflectionfeatures are anti-reflection rows.
 26. The method of claim 25, whereinthe anti-reflection rows have substantially triangular cross-sections.27. The method of claim 24, wherein the anti-reflection features areformed by etching the back surface with a selective wet etchingsolution.
 28. The method of claim 24, wherein the anti-reflectionfeatures are formed by etching the back surface withphotoelectrochemical etching.
 29. The method of claim 28, wherein theanti-reflection features are anti-reflection rows; and wherein theanti-reflection rows are formed by scanning the back surface, in thepresence of a photoelectrochemical etching solution, with a line-focusedlaser beam such that the etching rate is the highest when the speed ofthe line-focused laser beam is the slowest and the etching rate is thelowest when the speed of the line-focused laser beam is the fastest. 30.The method of claim 29, wherein the step of scanning the back surfacecomprises the step of varying the speed of the line-focused laser beamso as to create anti-reflection features having substantially triangularcross-sections.
 31. The method of claim 29, further comprising:fabricating a VCSEL on the semiconductor substrate; and wherein theanti-reflection features reduce specular reflection of light into theVCSEL.
 32. An apparatus for conditioning a semiconductor substratehaving a back surface, in the presence of an etching solution, theapparatus comprising: a line-focused laser beam generator for generatingand applying a line-focused laser beam to the back surface of thesemiconductor substrate; a holder positioned under the line-focusedlaser beam generator, wherein the holder holds the semiconductorsubstrate in place while the semiconductor substrate is beingconditioned; and a controller communicably linked to the line-focusedlaser beam generator, wherein the controller controls the relativemovement between the line-focused laser beam generator and the holder soas to form anti-reflection rows on the back surface of the semiconductorsubstrate.
 33. The apparatus of claim 32, wherein the anti-reflectionrows have arbitrary cross-sections.
 34. The apparatus of claim 32,wherein the controller moves the line-focused laser beam generatorrelative to the holder.
 35. The apparatus of claim 32, wherein thecontroller moves the holder holding the semiconductor substrate relativeto the line-focused laser beam generator.
 36. The apparatus of claim 32,wherein a VCSEL is fabricated on the semiconductor substrate; andwherein the anti-reflection features reduce specular reflection of lightinto the active region of the VCSEL.